Porous metallic materials having a three-dimensional network skeleton in which pores are formed are conventionally used as structural members of various machines and apparatus. For example, such metallic materials are used as an active substance holding material of an electrode of an alkaline secondary battery, a hydrolytic electrode, a kerosene atomizing member of a petroleum heater, a magnetic shield packing, a gas expansion buffer of an air cushion using an explosive, a sound absorbing material, various filters such as a hydrolytic filter of a clarifier, an electrostatic filter of an air clarifier, an oil mist filter for an engine exhaust gas, a filter of a high-temperature exhaust dust collector, etc.
An example of known porous metallic materials is a material having a skeleton which is obtained by plating a spongelike resin and then burning the resin, and which has pores formed therein by burning, as disclosed in, for example, Japanese Patent Laid-Open Patent Application No. 5-6763. This porous metallic material has a specific surface area of 5 to 75 cm.sup.2 /cm.sup.3 and a porosity of 92 to 97%.
However, it has been required to increase the porosity and specific surface area of the porous metallic material with recent increases in the performance and output of various machines and apparatus and recent decreases in the weight thereof.
On the other hand, an electrode for an alkaline secondary battery such as a nickel-cadmium battery, a nickel-hydrogen battery or the like is produced by a method in which a spongelike porous metallic substrate having pores formed by three-dimensional continuous network connection of a skeleton consisting of a Ni metal is impregnated with a paste formed by kneading an active material powder with a viscous aqueous solution containing 1 to 2% of CMC (carboxymethyl cellulose) and PTFE (polytetrafluoroethylene), dried and then rolled.
The spongelike porous metallic substrate comprising the pores and the skeleton and used for producing an electrode for an alkaline secondary battery can be obtained by Ni-plating an urethane foam, and then burning the urethane foam. The spongelike porous metallic substrate obtained by burning the urethane foam has the skeleton having the pores formed by burning the urethane, and generally has an average pore size of 400 to 600 .mu.m, a porosity of 93 to 97% and a specific surface area of 40 cm.sup.2 /cm.sup.3.
For a nickel-cadmium battery of alkaline secondary batteries, nickel hydroxide powder and cadmium hydroxide powder are used as a positive electrode active material and a negative electrode active material, respectively. On the other hand, for a nickel-hydrogen battery, nickel hydroxide powder and hydrogen occluding alloy powder are used as a positive electrode active material and a negative electrode active material, respectively.
When a closed alkaline secondary battery containing a conventional electrode in which pores of a spongelike porous metallic substrate are filled with an active material is repeatedly charged and discharged, the characteristics thereof gradually deteriorate, and the service life is limited to 500 to 1000 times of charging and discharging operations. The deterioration in the closed alkaline secondary battery is caused by an increase in internal pressure due to exhaustion of an electrolyte in a separator. It is said that the exhaustion of the electrolyte is caused by solidification of the electrolyte due to corrosion of the alloy, a change in electrolyte distribution due to swelling of the positive electrode, etc.
Corrosion of the alloy includes corrosion of a metallic member such as a battery can and corrosion of the hydrogen occluding alloy of a nickel-hydrogen battery, and represents the phenomenon that a part of the electrolyte is solidified to produce a corrosion product due to corrosion, thereby decreasing the amount of the electrolyte.
Swelling of the positive electrode is caused by the phenomenon that, when nickel hydroxide as the positive electrode active material is changed to --NiOOH through a crystal structure --NiOOH and --Ni(OH).sub.2 by repetition of the charging and discharging operations, respectively, micropores referred to as "mesopores" are formed in the particle surfaces of the positive electrode active material powder, the electrolyte permeates into the mesopores due to the capillary phenomenon, and thus unevenly distributed in the positive electrode, thereby decreasing the amount of the electrolyte in the separator.
Since the conventional alkaline secondary battery does not have a sufficient life, as described above, an alkaline secondary battery having a longer life is demanded.
It is also known that the capacity of the closed alkaline secondary battery containing an electrode increases as the amount of the active material contained in the electrode increases. However, in an electrode comprising the spongelike porous metallic substrate having pores which are formed by three-dimensional continuous connection of the conventional skeleton consisting of a Ni metal, since the pores formed in the skeleton by burning are closed and filled with no active material, the amount of the active material contained in the electrode is limited. There is thus demand for an electrode containing an increased amount of active material.